This invention relates to apparatus for determining the physical characteristics of materials and, in particular, to an apparatus for determining the rate at which a particular solution will etch a nonconductive material. The invention is particularly useful in the production of printed circuit boards.
Printed circuit board manufacture requires chemical cleaning of epoxy smear from drilled holes in the printed circuit board prior to copper plating. Typically, the smear of epoxy that finds it way into the drilled holes is removed by immersing the printed circuit board in a chemical solution that dissolves or etches the epoxy material. It is necessary to precisely monitor and control the etch rate of the epoxy so that there is consistent quality of the final product. One method presently used to monitor the etch rate is to perform a manual gravimetric analysis that is accomplished by weighing a small piece of the epoxy material, known in the art as a coupon, before and after a timed immersion in the etching solution. While this method does provide data from which the etech rate of the epoxy in the solutoin can be determined, it requires several steps and calculations, which makes the method slow, labor-intensive and not easily adaptable to on-line automated operation.
It is advantageous to provide a system in which the etch rate of the material in the solution can be tracked over a period of time by automatic actuation of several test probes periodically over a length of time. It is therefore an object of the present invention to provide a method and apparatus in the form of a test probe adaptable to determine the etch rate of nonconductive materials in an etching solution. The probe is configured to provide sufficient data to enable etch rate determination with a minimum of mathematical calculation and data recording. Also it is desirable to provide a measurement system that is easily automated for repeated measurements occurring over a finite length of time.